Energetics and dynamics of threshold photoion-pair formation in HF/DF

Threshold ion-pair production spectroscopy (TIPPS) has been applied to two isotopomers, HF and DF. From the high resolution (similar to 0.3 cm(-1)) TIPP spectra, the ion-pair thresholds of HF/DF have been precisely measured. Combined with the ionization energy of H(D), the electron affinity of F, and the zero point energies of HF/DF, the difference between their classical bond dissociation energies was obtained as D-e(H-F)-D-e(D-F)=12.4 +/- 0.5 cm(-1). Our result provides an experimental estimate of the Born-Oppenheimer breakdown in the ground electronic state. The present work also measured the total ion-pair yield spectra of HF and DF in the threshold region, and the ion-pair formation mechanisms of these two molecules were discussed in light of the high resolution results.